Abstract:
In this paper we study bulk viscosity in a thermal QCD model with large
number of colors at two extreme limits: the very weak and the very strong 't Hooft
couplings. The weak coupling scenario is based on kinetic theory, and one may go to
the very strong coupling dynamics via an intermediate coupling regime. Although the
former has a clear description in terms of kinetic theory, the intermediate coupling
regime, which uses lattice results, su ers from usual technical challenges that render
an explicit determination of bulk viscosity somewhat di cult. On the other hand,
the very strong 't Hooft coupling dynamics may be studied using string theories at
both weak and strong string couplings using gravity duals in type IIB as well as
M-theory respectively. In type IIB we provide the precise
uctuation modes of the
metric in the gravity dual responsible for bulk viscosity, compute the speed of sound
in the medium and analyze the ratio of the bulk to shear viscosities. In M-theory,
where we uplift the type IIA mirror dual of the UV complete type IIB model, we
study and compare both the bulk viscosity and the sound speed by analyzing the
quasi-normal modes in the system at strong IIA string coupling. By deriving the
spectral function, we show the consistency of our results both for the actual values of
the parameters involved as well for the bound on the ratio of bulk to shear viscosities.
